Point contact semiconductor forming method



March 1, 1960 VOLT/46:6 CURRENT cane-era)? o R. ZULEEG 2,926,418

POINT CONTACT SEMICONDUCTOR FORMING METHOD Filed Aug, 19, 1955 COLLECTOR o SIGNAL 00 FIG-l 1 m I g l I nmam E STE'ADY smrs coup/nous Q l 4 .22 cawmau pun/Ive PULSE v u a' TIME \l V M PULS'E i |DURATION I THGRMHL I DIFFUSION UMD'R ELEVATED WWW I l I l I 43 INVENTOR. nae. RAM/ER ZUL'EG HIS Arron/Ev United States Patent POINT CONTACT SEMICONDUCTOR FORMING METHOD Rainer Zuleeg, North Adams,

Electric Company, of Massachusetts Mass, assignor to Sprague North Adams, Mass., a corporation This invention relates to point contact semiconductive devices and more particularly to a process for improving the uniformity of amplification characteristics by the formation of the point contact.

A typical point contact semiconductive device consists of at least two rectifying contacts, of which one is a metal point contact, placed in close spacing on a semiconductive crystal to which a third electrode, the base or ohmic connection, is made. The properties of such a device, particularly its amplification characteristics, can be improved materially by a technique known as formation. The formation consists generally of pass ing an electrical discharge between the point contact and the semiconductor crystal. Various formation techniques have been described in the literature, for example, United States Patent No. 2,263,829 and British Patent No. 724,044. However, the units formed by known procedures vary substantially in amplification characteristics from unit to unit. These variations in the electrical characteristics of these point contact transistors can be often traced to the substantial variations in the many parameters of the device, such as the shape of the electrode points, force of the electrodes on the crystal surface, resistivity of the crystal, separation between the electrodes, and the general surface condition of the crystal which is contacted by the electrode, and the prior forming techniques yield different results with differences in parameters listed above.

It is therefore an object of this invention to overcome the foregoing and related disadvantages of the prior art processes of producing point contact transistors. It is a still further object of this invention to produce by a novel process point contact transistors having substantial uniformity of amplification characteristics. It is a still further object of this invention to form a metal point contact by a process independent of limited variations in the many transistor parameters of device referred to above. .Other objects of this invention will be apparent from the following specification and appended drawings.

In general, the objects of this invention have been achieved by the method of improving the amplification of a semiconductor amplifier having rectified contacts on a semiconductive body, comprises producing a constant current pulse having a current density greater than 100 amperes/sq. centimeter which flows between the semiconductive body and the metal point contact pressed against it. The constant current pulse is produced by a high voltage to force electrons into the semiconductive body, which voltage has a wave form having an initial portion of high intensity followed by a longer portion of low intensity. To fully understand the scope of the novel concept of thisinvention reference should now be made to the appended drawings in'which:

Fig. 1 portrays in diagrammatic form a point contact transistor with circuit means connected thereto for carrying out the forming operation of this invention; and

Fig. 2 shows voltage and current curves useful in explaining the invention.

Referring now to Fig. l, a point contact transistor of the usual type has an ohmic base electrode 12 attached to the semiconductor crystal 14 and on the opposite face two point contact electrodes 16 and 18, the former being an emitter and the latter a collector. In accordance with this invention a high energy level constant current pulse is applied to one of the point contact electrodes, the collector 18 being preferred in the case of an n-type semiconductor material such as germanium doped with a minor amount of antimony. The constant current source is shown in this diagrammatic presentation of Fig. 1 as a pulse amplifier circuit utilizing a triode 20, however it is preferred in actual operation to use a pentode, the latter having a more constant current-voltage relationship. The plate 22 of the vacuum tube is connected to the collector 18 by means of a capacitor 24 and in the absence of a signal imposed upon the grid of the tube 20 the latter is biased to cut-off. The cathode 26 is connected to the base electrode '12 and both are grounded. During forming, the collector is kept under a negative bias current by means of the high impedance constant current source 33 to avoid sparking, at the moment of the high voltage, high'current pulse. The plate electrode of tube 20 also is connected to a high positive potential DC. voltage source having a value of from 400 to about 1200 volts. A load resistor 28 is inserted in the plate circuit. When it is desired to form the collector, a positive pulse or input signal as illustrated at the right side of tube 20 in Figure 1 is placed upon the control grid 30 so as to render the tube conducting and retain it as such for the entire desired duration of the constant current pulse of energy which is to be dissipated between the collector 18 and the body of the crystal 14 during formation. The pulse is of a duration of from about 50 microseconds and upward so that it can be produced by known radar circuitry. The pulse can be from a pulse-forming network, multivibrator or by clipping and peaking circuits in a low power driver stage. When the tube 20 is converted to a conducting state, a very high negative potential or collector signal pulse as illustrated above collector 18 in Figure 1 is immediately imposed on the collector bringing about the rapid breakdown of the point contact resistance between the collector point and the crystal :14. This resistance thereafter remains low and a constant current is dissipated in the crystal 14 for the remainder of the pulse. Although this technique produces the uniformity of amplification characteristics hitherto unachieved in usual procedures associated with the formation ofpoint contact electrodes, it is found that even superior results obtain when the emitter is biased in'the forward direction prior to imposition of the current pulse on the collector. The positive bias comes from a high impedance source 32 and it should be in such magnitude as to cause in a typical point contact transistor approximately 3 milliamperes of current to flow between the emitter and the base electrode 12. During the period of the current pulse being imposed upon the collector the current flow in the emitter-collector circuit is allowed to decay by means of the time constant network set up with resistor 34 and capacitor 36. Their values are chosen so as to achieve a proper timecon"- stant, to allow this decay of the emitter current to'substantially zero during the period of the application of the current pulse energy to the collector 18, 3 2 and 33 can be, for example, pentodes biased into the constant current conduction region. The change in the crystal caused by the formation processes are further shown in an expanded diagram;

matic presentation in Fig. 1 as two parts, an n-region 38 which region is attained through impurity diffusion from the collector and a p-region 39 which is attained by the thermal conversion due to the dissipation of the electrical energy in the crystal during formation step. It has been found thatby following procedures set'forth herein, the dimension of the'p-region can be very closeiy controlled, which in turn defines improvedaud uniform amplification characteristics from point contact transistors produced in this manner.

In Fig. 2-are shown several curves which indicate the current and voltage relationships of the formation process. Briefly, all the curves are broken into 3 periods: the steady state prior to pulse formation 1, formation period II and steady state after formation III. It can be seen that as the high energy pulse is imposed upon ithat segmentof the circuit of Fig. 1 the collector voltage 40 rises sharply to 41, remains almost constant until the point contact resistance markedly decreases. This decrease drops the collector voltage from 42 to 53 where .itiremains for the duration of the pulse period 'of the constant current which is imposed in the crystal 14 through the collector 18. The point 42 may shift slightly :to the left or right dependent upon the time required for breakdown of the point contact resistance, however the total period for the breakdown of the point contact resistanceis in the order of microseconds, which is a very small portion of the entire pulseperiod. The collector current 44 is much larger than the emitter current 46, the former in the order of 0.1 ampere, while the latter is in the order of 3 milliarnperes for the conventional point contact transistor amplifier. The collectorcurrent wave form approaches a square Wave and in contrast to the procedures of the prior art is of a constant'natnre for a known very short period of time. The-emitter current on the other hand is shown to decay from its initial value to a slightly negative value during the period of the formation pulse of the current. a

In the discussion of the invention the application of a formation voltage has emphasized that the current dissipated within the crystal and flowing through the collector should be of a constant magnitude during a relatively short period. On the other hand, the voltage :must be of such magnitude as to cause the point con- "tact resistance to drop off during the initial portion of the ,pulse period and to manifest itself in a wave form having a very short initial period of high voltage and a lower constant voltage for the major part of the cycle. lt'should be understood that the limitation imposed upon "the voltage and current or current density is applicable bnly to the small point contact type of transistors having excellent amplification characteristics. The general techniques set forth are applicable not only to such devices, but also to larger devices where the voltage ranges may differ as may the current amplitudes, so for purposes of defining the true scope of this invention the current can be readily set forth asa current density. To obtain suitable formation the current density must be atleast about 100 amps/cmfi'and not greater than 2000 amperes/cmfi'computed on the cross-sectional area of the metal point contact. Above about 2000 amperes/ cm. the pulse periods are so short as to make quality control on automatic apparatus difiicult. To achieve suitable breakdown of point contact resistance within a short period of time it has been found that the voltage .levelshould be at least 400 v. and preferably within the range of about 400 v. to about 1500 v. Suitable formation as shown is dependent upon the energy dis- "sip'atedin the crystal to effect thermal breakdown of the semiconductor to a region having a conductivity opposite to that "of the bulk of the crystal. For this "reason the "formation period for the diffusion of the impurities must not exceed a certain critical time. For reasons for defining this it has been found that about 's'o microseconds is the minimum period over which the pulse must be imposed so as to achieve suitable formation. The pulse period can extend upwards of 1 millisecond, however the maxirnum duration of the pulse is generally dictated by the various parameters such as resistance of the crystal, type of impurities, as well as the spacing of the point contacts.

In this specific example of the procedure of the invention a crystal was produced of antimony doped germanium having a resistivity of 4-5 ohm-centimeters. As the point contact electrodes, the emitter was fabricated from beryllium copper, while the collector was of Phosphor bronze having a phosphorus content of between 02 and 0.3%. The spacing of the point contacts was about 2 mils. The surface of the crystal upon which the point contacts were disposed was etched for 4 min-- utes with a mixture of hydrofluoric acid, hydrogen peroxide and water in volume proportion of 1-1-4 and thereafter dried with a heat lamp for 5 minutes under an infra-red lamp. With emitter current equal to 3 milliamperes and the collector current equal to 4 milliamperes, the collector voltage being between '40 and 60 volts, a single pulse of voltage was used to form the collector. The voltage had an initial pulse amplitude of 800 v. which dropped after a period of 10-15 microseconds to approximately 10-20 volts and the pulse was continued for a full period of microseconds. The current density of the constant current pulse based on the area of the collector was 500 amps/cm? The constant current pulse generator was a SD21 tube biased to l50 volts in the formation pulse controlled by a 100 microsecond pulse of 450 volts imposed upon the grid. A positive potential of 600 V. DC. was imposed on resistor 28, and hence on plate 22 during the nonconducting condition of tube 20. The capacitor 24 was of 20 microfarad capacity and the time constant network of resistor 34 and capacitor 36 was 100 microseconds.

Although the, process of the invention has been shown with point contact devices having a plurality of metal rectifying contacts, it extends to all semiconductive devices having at least one of said contacts.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope hereof, it is to be understood that the invention is not limited to the specific embodiments hereof except as defined in the appended claims.

What is claimed is:

1. The method of forming a germanium transistor having a point-contact collector electrode, which method is characterized by the step of applying between the collector and the germanium an isolated reverse current pulse at least about 50 microseconds long, the pulse having a voltage with an initial value of from 400 to 1500 volts and after barrier breakdown at the collector point, drops to a level such that the current remains about constant for the'entire pulse.

2. The method of improving the amplification of a semiconductor amplifier having a point-contact electrode pressed against a semiconductive body, which method comprises producing a substantially constant current pulse between said electrode and said body, said pulse having a voltage wave form with an initial portion of high intensity followed by a longer portion of low intensity.

References Cited in the file of this patent UNITED STATES PATENTS 2,577,803 Pfann Dec. .11, "1 951 2,589,658 Bardeen et a1. Mar. 12,1932 2,752,661 Allerton et a1. July 3, 1956 2,784,478 Slade Mar. 12, 1957 OTHER REFERENCES Transistor Technology, vol. 1, pp. 466-475, pub. by Bell Tel. Laboratories, 1 952. 

1. THE METHOD OF FORMING A GERMANIUM TRANSISTOR HAVING A POINT-CONTACT COLLECTOR ELECTRODE, WHICH METHOD IS CHARACTERIZED BY THE STEP OF APPLYING BETWEEN THE COLLECTOR AND THE GERMANIUM AN ISOLATED REVERSE CURRENT PULSE AT LEAST ABOUT 50 MICROSECONDS LONG, THE PULSE HAVING A VOLTAGE WITH AN INITIAL VALUE OF FROM 400 TO 1500 VOLTS AND AFTER BARRIER BREAKDOWN AT THE COLLECTOR POINT, DROPS TO A LEVEL SUCH THAT THE CURRENT REMAINS ABOUT CONSTANT FOR THE ENTIRE PULSE. 